It won't change much though. I have a married friend, who has married in her status. She had a PM box message stating she was married and not looking to flirt. Doesn't stop her from getting hit on. One guy she had a chat with, thinking he was just being friendly, when he came on the second time and had a chat with her he had written her a poem. So if saying you're married and not interested in anything with any guy doesn't stop a man, neither is 'engaged' or 'in a relationship'.
Not to say its still a bad idea. It will help to some degree, with guys who are actually decent guys and looking for a real relationship. They'll know not to go sniffing in your direction. But it won't stop all of them.